Multistate magnetic memory element using metamagnetic materials

ABSTRACT

A metamagnetic tunneling-based spin valve device for multistate magnetic memory comprising an electronic memory logic element with four stable resistance states. A metamagnetic tunneling-based spin valve device for multistate magnetic memory comprising a layer of a metamagnetic material, a layer of a nonmagnetic material on the layer of a metamagnetic material, and a layer of a ferromagnetic material on the layer of a nonmagnetic material. A method of making a metamagnetic tunneling-based spin valve device for multistate magnetic memory.

REFERENCE TO RELATED APPLICATION

This application is a non-provisional of, and claims priority to and thebenefits of, U.S. patent application Ser. No. 62/662,967 filed on Apr.26, 2018, the entirety of which is hereby incorporated by reference.

BACKGROUND

This invention disclosure describes and demonstrates a process that usesthin films of metamagnetic materials, such as FeRh alloys, in magneticmultilayers for memory purposes.

This invention disclosure describes and demonstrates a method forstabilizing the resistance of the multilayer in any of four unique anddiscernable states by magnetic field and operational temperaturecontrol.

This invention disclosure describes and demonstrates a method using ametamagnetic/magnetic metallic multilayer for low-resistance, high-speedapplications.

This invention disclosure describes and demonstrates a method thatincorporates insulating tunnel barriers between a metamagnetic andmagnetic layer for increased switching sensitivity via a large tunnelingmagnetoresistance response.

Magnetic random-access memory (MRAM) is a promising technology with thepotential to take over a large segment of the memory market currentlyserved by electronic charge-based devices. The appeal of magnetic-basedmemory is the built-in non-volatility of the memory state using themagnetization of a magnetic layer. Furthermore, magnetic memory isconsidered to have an unlimited amount of memory cycles, is scalable,and is fast.

All of these features are recognized by industry as promising metrics tomeet the growing need for memory solutions in the future. Even thoughMRAM is available, the field is still in its infancy.

The current state of the art magnetic memory element is comprised offerromagnetic layers separated by an insulator such as MgO. There areonly two available stable states—the parallel or antiparallel alignmentof the two ferromagnetic layers.

Alternatively, one can use the anisotropic magnetoresistance effect of asingle metamagnetic layer such as FeRh. However, the AMR effect reportedin literature is ˜0.2% and has a theoretical maximum of ˜1.5%.Charge-based devices for memory purposes are either fast and volatile(SRAM) or slow and non-volatile with limited repeatability (“Flashmemory”). Other alternative memories, such as, resistance-based phasechange materials only have two stable states (e.g. amorphous vscrystalline Ge) or ion movement in memristors.

Films of iron rhodium (FeRh) are known to exhibit a uniqueantiferromagnetic (AF) to ferromagnetic (FM) transition slightly aboveroom temperature, known as the metamagnetic phase transition.

FeRh is a unique material that changes its intrinsic magnetic order atan ambient temperature range of 280 K to 360 K. This highly unusualmetamagnetic transition offers the possibility to switch between the twomagnetic phases by external perturbation, such as temperature, offeringcompletely new avenues for magnetism-based device design.

SUMMARY OF DISCLOSURE Description

This invention describes and demonstrates a process that uses thin filmsof metamagnetic materials, such as FeRh alloys, in magnetic multilayersfor memory purposes.

This invention describes and demonstrates a method for stabilizing theresistance of the multilayer in any of four unique and discernablestates by magnetic field and operational temperature control.

This invention describes and demonstrates a method using ametamagnetic/magnetic metallic multilayer for low-resistance, high-speedapplications.

This invention describes and demonstrates a method that incorporatesinsulating tunnel barriers between a metamagnetic and a magnetic layerfor increased switching sensitivity via a large tunnelingmagnetoresistance response.

DESCRIPTION OF THE DRAWINGS

The following description and drawings set forth certain illustrativeimplementations of the disclosure in detail, which are indicative ofseveral exemplary ways in which the various principles of the disclosuremay be carried out. The illustrated examples, however, are notexhaustive of the many possible embodiments of the disclosure. Otherobjects, advantages and novel features of the disclosure will be setforth in the following detailed description when considered inconjunction with the drawings.

FIG. 1 illustrates a timeline of magnetoresistance devices. A GMR devicefrom Fe/Cr/Fe multilayer. Fe layers are parallel, one spin speciesexperiences weak scattering only in both magnetic layers and a lowoverall resistance is measured. Fe layers are anti parallel, both spinspecies experience strong scattering and a high resistance is measured.

FIG. 2 illustrates a simplified diagram of tunneling from ferromagnet toferromagnet. Magnetization is parallel. One spin species tunnels from alarge density of states to a large density of states, and a low tunnelresistance is measured. Magnetizations antiparallel, both spin speciestunnel from a large (small) density of states to a small (large) densityof states, and a high tunnel resistance is measured.

FIG. 3 illustrates a graph of resistance vs. magnetic field of a MTJ,minor loop is shown. Also illustrated is resistance vs. magnetic fieldof a metallic pseudo spin valve, major loop is shown. Arrows indicatemagnetization direction of top and bottom layer.

FIG. 4 illustrates saturation magnetization curves as a function oftemperature for FeRh(Pd) single layer films with 1 T applied field grownat 600° C. with varying thicknesses: 50 nm, 30 nm and 10 nm. Alsoillustrated is the 4-point resistance of NRL grown FeRh film vs.Temperature. Arrows indicate the temperature sweep direction.

FIG. 5 illustrates magnetoresistance curves above and below the phasetransition.

FIG. 6 illustrates major and minor loops at a fixed temperature, showingfour stable resistance states, or operational regimes, of the device ata fixed temperature.

DETAILED DESCRIPTION OF THE INVENTION

This invention disclosure describes and demonstrates a process that usesthin films of metamagnetic materials, such as FeRh alloys, in magneticmultilayers for memory purposes.

This invention disclosure describes and demonstrates a method forstabilizing the resistance of the multilayer in any of four unique anddiscernable states by magnetic field and operational temperaturecontrol.

This invention disclosure describes and demonstrates a method using ametamagnetic/magnetic metallic multilayer for low-resistance, high-speedapplications.

This invention disclosure describes and demonstrates a method thatincorporates insulating tunnel barriers between a metamagnetic andmagnetic layer for increased switching sensitivity via a large tunnelingmagnetoresistance response.

One of the first successful commercial transitions in spintronics wasbased on the giant magnetoresistance (GMR) effect, shown in FIG. 1,discovered in 1988. A nonmagnetic metal layer separates two magneticlayers. Electrons are naturally spin-polarized in a ferromagnet (FM) dueto its spin-polarized density of states. Majority spin electrons canhave their moment (spin) aligned parallel or antiparallel to themagnetization of the FM. These electrons experience weak scatteringduring transport within the FM. Minority spin electrons have theirmoment (spin) aligned either anti-parallel or parallel to themagnetization and experience stronger scattering in the FM.

Example 1

If the magnetizations of the two magnetic layers are parallel, asdepicted in FIG. 1, then the majority spin electrons of the top layerwill enter the bottom layer as majority spin and the overall resistanceis low. When the two layer magnetizations are aligned anti-parallel, asdepicted in FIG. 1, the majority spins of the top layer will be minorityspins in the bottom layer, and the overall stack resistance will behigher.

Resistance changes on the order of 10% were obtained with thesestructures, which was far better than the best anisotropicmagnetoresistance (AMR) devices that were used as the read-heads todetect the small changes in magnetic field lines on the magnetic platterof a hard disk at the time.

Example 2

Later work replaced the metallic interlayer of the GMR stack by a thininsulator. Transport across this thin insulator is possible bytunneling, a purely quantum mechanics-based effect. Tunneling across athin insulator depends critically on the density of states (DOS)available on both sides of the tunnel barrier.

Since the DOS at the FM/insulator interface is spin dependent, thetunnel process becomes spin dependent. When the magnetic layers arealigned with their magnetizations parallel, majority electrons that havea large DOS in the left electrode, as in FIG. 2, can readily tunnel tothe large DOS of the majority state of the right electrode. However,when the magnetic electrodes have their magnetizations anti-parallel,the majority electrons with a large DOS can only tunnel to the minoritystates of the right electrode, which has a much smaller DOS.

Thus, the tunnel resistance is increased by this spin-dependent effect.Tunnel magnetoresistance effects of over 200% have been observed inFe/MgO/Fe tunnel barriers. These barriers are now the major component inread-heads for hard disks and they are the memory elements of choice incurrently available MRAM.

As discussed above, a magnetic tunnel junction and metallic spin valvehave two states, parallel and anti-parallel, with two significantlydifferent resistances, as shown in FIG. 3. When this property is used tobuild MRAM, this memory is non-volatile, since the parallel and theanti-parallel states are stable states.

Films of iron rhodium (FeRh) have long been known to exhibit a uniqueantiferromagnetic (AFM) to ferromagnetic (FM) transition slightly aboveroom temperature, the metamagnetic phase transition. FeRh is a uniquematerial that changes its intrinsic magnetic order at an ambienttemperature of 360 K. This highly unusual metamagnetic transition offersa possibility to switch between the two magnetic phases by externalperturbation.

Example 3

FIG. 4 demonstrates the drastic change in magnetization (M) when an FeRhfilm is driven from its AFM phase to FM phase and back by temperaturecycling of the sample.

The magnetic phase exhibits hysteresis while temperature cycling thesample, as it takes more energy to change back from one phase to theother.

This magnetic phase change is also seen in the temperature dependence ofthe resistance, since the spin dependent scattering in the AFM phase ishigher than it is in the FM phase. FIG. 4 shows the temperaturedependent resistance of a NRL grown FeRh film, where a similarhysteresis is observed as in the magnetization versus temperature curve.

Example 4

Fabrication of FeRh Tunneling-based Spin Valve Device is described.

We demonstrated the fabrication of devices with one of the ferromagneticlayers of a magnetic tunneling junction (MTJ) or a metallic spin valvereplaced by a metamagnetic layer, such as FeRh.

In order to demonstrate the principle function of this device, wefabricated the following multilayers, 30 nm FeRh/5 nm Cu/30 nm NiFe onMgO and 30 nm FeRh/5 nm Cu/30 nm NiFe on sapphire.

These films were grown by sputter deposition. The FeRh with thickness 30nm was grown at 600° C. Then, to minimize interdiffusion, the structurewas allowed to cool down to room temperature before the 5 nm Cu and 30nm NiFe layers were sputtered.

Example 5

We demonstrated the operation of the above fabricated device in threedistinct temperature regimes comprising four distinct operationalregimes.

First, at “cold” temperatures, as defined as being below the phasetransition (for this sample <360K), FeRh film is in its AFM phase and nospin-valve effect is expected to be measured, as shown in the bottomcurve in FIG. 5.

The only observable magnetoresistance effect is the anisotropicmagnetoresistance at the coercive fields of the NiFe film.

Second, at “warm” temperatures, as defined as being above the phasetransition (for this sample >360K), FeRh is in its FM phase and thedevice is expected to work as a traditional spin valve device, as shownin the top curve of FIG. 5.

Example 6

A distinct regime was observed when the device was operated at a fixedtemperature in the hysteretic part of the magnetic phase change.

Here, the device operated as a spin-valve device in two differentresistance regimes (giving the third and fourth operational regimes)that can be accessed by either cooling through the magnetic phase changeor heating up through the phase change.

This is evident in FIG. 6. FIG. 6 shows the major magnetoresistance“loops” at a fixed temperature and FIG. 6 shows the minor “loops.” Herethe term “loops” refers to a complete cycling of the magnetic field.

Example 7

FIG. 6 shows the four stable resistance states at a fixed temperatureand zero magnetic field that can all be individually addressed bychanging two physical quantities, magnetic field and temperature.

The device shown in FIG. 6 is a metallic spin valve with ˜1.5% magnitudechange in magnetoresistance above the zero-field value, hereafterreferred to as “MR.” Metallic spin valves have been shown with effectsof up to ˜10% MR and the magnetic behavior can be tuned with the spacerthickness and composition and the FM layer thickness and composition.

Example 8

The metallic spacer layer can be replaced with an insulating tunnelbarrier in our demonstrated device to make an MTJ, which is easilyachievable after our demonstration above. MTJs can havemagnetoresistance effects of ˜200% MR for certain combinations of FMsand insulators.

The device shown in FIG. 6 uses FeRh as the metamagnetic layer andtemperature to drive the magnetic phase change. Laser heating andcooling can be used to drive the magnetic phase change and externalmagnetic fields can set the magnetization of the FM layer.

Our multistate magnetic memory element using metamagnetic materials hasmany advantages.

For example, the magnetic device can be switched on and off by operatingit below or above the phase change. No other currently envisioned ormarketed magnetic-based devices use this approach, nor can they do this.

Four stable resistance states can be obtained at a fixed temperature andzero magnetic field that can all be individually addressed by changingtwo physical quantities: magnetic field and/or temperature. This issuperior to the current magnetic-based, charge-based or structuralphase-change memory devices which only have two stable states.

Further, our device can be an ad-hoc or “drop-in” replacement asmagnetic memory element allowing 2 bits of information per memoryelement. No other similar device technology exists that is capable ofthis performance.

Additionally, our device can be used as a single failsafe electronicswitch that needs two physical properties (temperature and magneticfield) to switch its electronic state, thus adding an extra failmechanism over electronic switches that require only one physicalproperty to change state, such as charge (transistor), magnetic field(magnetic tunnel junction), temperature (phase change element), or Ionmovement (memristor).

The above examples are merely illustrative of several possibleembodiments of various aspects of the present disclosure, whereinequivalent alterations and/or modifications will occur to others skilledin the art upon reading and understanding this specification and theannexed drawings. In addition, although a particular feature of thedisclosure may have been illustrated and/or described with respect toonly one of several implementations, such feature may be combined withone or more other features of the other implementations as may bedesired and advantageous for any given or particular application. Also,to the extent that the terms “including”, “includes”, “having”, “has”,“with”, or variants thereof are used in the detailed description and/orin the claims, such terms are intended to be inclusive in a mannersimilar to the term “comprising”.

What we claim is:
 1. A method of making a metamagnetic tunneling-basedspin valve device for multistate magnetic memory comprising the stepsof: providing a substrate; fabricating a layer of a metamagneticmaterial on the substrate; fabricating a layer of a nonmagnetic materialon the layer of a metamagnetic material; and fabricating a layer of aferromagnetic material on the layer of a nonmagnetic material.
 2. Themethod of making a metamagnetic tunneling-based spin valve device formultistate magnetic memory of claim 1 wherein the layer of ametamagnetic material comprises FeRh.
 3. The method of making ametamagnetic tunneling-based spin valve device for multistate magneticmemory of claim 2 wherein the layer of a ferromagnetic materialcomprises NiFe.
 4. The method of making a metamagnetic tunneling-basedspin valve device for multistate magnetic memory of claim 3 whereinlayer of a nonmagnetic material comprises a nonmagnetic metal.
 5. Themethod of making a metamagnetic tunneling-based spin valve device formultistate magnetic memory of claim 4 wherein the nonmagnetic metalcomprises copper.
 6. The method of making a metamagnetic tunneling-basedspin valve device for multistate magnetic memory of claim 1 wherein thestep of fabricating a layer of a metamagnetic material on the substratecomprises the step of sputtering or depositing by sputter deposition thelayer of a metamagnetic material on the substrate.
 7. The method ofmaking a metamagnetic tunneling-based spin valve device for multistatemagnetic memory of claim 6 wherein the step of sputtering or depositingby sputter deposition the layer of a metamagnetic material on thesubstrate comprises sputtering a layer of FeRh.
 8. The method of makinga metamagnetic tunneling-based spin valve device for multistate magneticmemory of claim 7 wherein the layer of FeRh is 30 nm thick.
 9. Themethod of making a metamagnetic tunneling-based spin valve device formultistate magnetic memory of claim 8 wherein the step of sputtering ordepositing by sputter deposition the layer of FeRh occurs at atemperature of 600° C.; and further comprising the step of cooling thelayer of FeRh to room temperature; wherein the step of fabricating alayer of a nonmagnetic material on the layer of a metamagnetic materialcomprises fabricating a layer of Cu; and wherein the step of fabricatinga layer of a ferromagnetic material on the layer of a nonmagneticmaterial comprises fabricating a layer of NiFe.
 10. A metamagnetictunneling-based spin valve device for multistate magnetic memorycomprising: a layer of a metamagnetic material; a layer of a nonmagneticmaterial on the layer of a metamagnetic material; and a layer of aferromagnetic material on the layer of a nonmagnetic material.
 11. Themetamagnetic tunneling-based spin valve device for multistate magneticmemory of claim 10 wherein the layer of a metamagnetic materialcomprises FeRh.
 12. The metamagnetic tunneling-based spin valve devicefor multistate magnetic memory of claim 11 wherein the layer of anonmagnetic material on the layer of a metamagnetic material comprises anonmagnetic metal.
 13. The metamagnetic tunneling-based spin valvedevice for multistate magnetic memory of claim 12 wherein thenonmagnetic metal comprises Cu.
 14. The metamagnetic tunneling-basedspin valve device for multistate magnetic memory of claim 13 wherein thelayer of a ferromagnetic material on the layer of a nonmagnetic materialcomprises NiFe.
 15. The metamagnetic tunneling-based spin valve devicefor multistate magnetic memory of claim 14 further comprising a MgOsubstrate or a sapphire substrate.
 16. The metamagnetic tunneling-basedspin valve device for multistate magnetic memory of claim wherein thelayer of FeRh is 30 nm thick; wherein the layer of Cu is 5 nm thick; andwherein the layer of NiFe is 30 nm thick.
 17. A metamagnetictunneling-based spin valve device for multistate magnetic memorycomprising: an electronic memory logic element with four stableresistance states.
 18. The metamagnetic tunneling-based spin valvedevice for multistate magnetic memory of claim 17 wherein the electronicmemory logic element with four stable resistance states comprises stateswhich depend on two physical domains.
 19. The metamagnetictunneling-based spin valve device for multistate magnetic memory ofclaim 18 wherein the two physical domains comprise temperature sweepdirection and magnetic field.
 20. The metamagnetic tunneling-based spinvalve device for multistate magnetic memory of claim 17 wherein theelectronic memory logic element with four stable resistance statescomprises two magnetic layers and wherein the two magnetic layers areseparated by a nonmagnetic layer.
 21. The metamagnetic tunneling-basedspin valve device for multistate magnetic memory of claim 20 wherein oneof the magnetic layers comprises FeRh; and wherein the nonmagnetic layercomprises a metal or a tunnel-barrier.
 22. The metamagnetictunneling-based spin valve device for multistate magnetic memory ofclaim wherein the electronic memory logic element with four stableresistance states comprises FeRh; wherein the FeRh undergoes a magneticphase change with temperature cycling and comprises an associatedhysteresis; and wherein the 4 stable resistance states are obtained at afixed temperature and zero magnetic field.
 23. The metamagnetictunneling-based spin valve device for multistate magnetic memory ofclaim wherein the four stable resistance states are individuallyaddressable with temperature cycling and magnetic field cycling; whereinthe resistance depends on the relative magnetization of magnetic layers;and wherein the metamagnetic tunneling-based spin valve device formultistate magnetic memory is used for nonvolatile electronics.